<<

Clim. Past, 3, 109–118, 2007 www.clim-past.net/3/109/2007/ © Author(s) 2007. This work is licensed of the Past under a Creative Commons License.

Predicting climate from vegetation in

C. Loehle National Council for Air and Stream Improvement, Inc., 552 S Washington St., #224, Naperville, IL 60540, USA Received: 22 September 2006 – Published in Clim. Past Discuss.: 23 October 2006 Revised: 8 January 2007 – Accepted: 8 February 2007 – Published: 12 February 2007

Abstract. at the have been 1 Introduction inferred from pollen assemblages, but these inferred climates are colder for eastern North America than those pro- The Pleistocene Last Glacial Maximum (LGM) period of duced by climate simulations. It has been suggested that low 18 000 ago has been widely interpreted as a of CO2 levels could account for this discrepancy. In this study bitter cold in eastern North America when and bo- biogeographic evidence is used to test the CO2 effect model. real forest extended hundreds of miles south of the sheets The recolonization of glaciated zones in eastern North Amer- and the temperate forest of the East retreated to the south- ica following the last ice produced distinct biogeographic ern coastal plain, to Florida, and westward into Texas and patterns. It has been assumed that a wide zone south of the Mexico (Davis, 1983, 1984; Davis and Shaw, 2001; Deevey, ice was tundra or parkland (Boreal-Parkland Zone or 1949; Delcourt and Delcourt, 1984, 1993; Jacobson et al., BPZ), which would have been recolonized from southern 1987; Maher et al., 1998; Maxwell and Davis, 1972; Over- refugia as the ice melted, but the patterns in this zone dif- peck et al., 1992; Prentice et al., 1991; Ritchie, 1987; Royall fer from those in the glaciated zone, which creates a major et al., 1991; Schoonmaker and Foster, 1991; Tallis, 1991; biogeographic anomaly. In the glacial zone, there are few en- Watts, 1970, 1971, 1973, 1979, 1980a, b; Watts and Stuvier, demics but in the BPZ there are many across multiple taxa. 1980; Webb et al., 1988, 1993; Whitehead, 1973; Wilkins In the glacial zone, there are the expected gradients of ge- et al., 1991). This reconstruction, which may be called the netic diversity with distance from the ice-free zone, but no standard model, is commonly presented in textbooks (e.g., evidence of this is found in the BPZ. Many races and re- Bradley, 1999; Delcourt and Delcourt, 1993; Pielou, 1991; lated species exist in the BPZ which would have merged or Ritchie, 1987; Tallis, 1991). The standard model is based hybridized if confined to the same refugia. Evidence for dis- largely on pollen and macrofossil records. These pollen tinct southern refugia for most temperate species is lacking. data have been interpreted qualitatively in some cases, and Extinctions of temperate flora were rare. The interpretation in other cases transfer functions or response surface models of as a boreal climate indicator may be mistaken over (e.g., Farrera et al., 1999; Nakagawa et al., 2002; Peyron et much of the region if the spruce was actually an extinct tem- al., 1998; Tarasov et al., 1999; Webb et al., 1993, 1997) have perate species. All of these anomalies call into question the been used to infer climate from pollen composition, with concept that climates in the zone south of the ice were ex- similar results. By any of these three methods, the inferred tremely cold or that temperate species had to migrate far to LGM climate is much colder than that simulated (Ganopol- the south. An alternate hypothesis is that low CO2 levels gave ski et al., 1998; Huntley et al., 2003; Kageyama et al., 2001; an advantage to and spruce, which are the dominant Pinot et al., 1999; Webb et al., 1993, 1997). Determining the trees in the BPZ, and to herbaceous species over trees, which correct paleotemperature is important for calibrating general also fits the observed pattern. Thus climate reconstruction circulation models (e.g., Crowley, 2000; Farrera et al., 1999; from pollen data is probably biased and needs to incorporate Ganopolski et al., 1998; Pinot et al., 1999). In addition, the CO2 effects. Most temperate species could have survived vegetation composition at this time is consistently described across their current ranges at lower abundance by retreating as having no analogs in modern periods. In this paper it is to moist microsites. These would be microrefugia not easily argued that both of these anomalies arise from a combination detected by pollen records, especially if most species became of ambiguous pollen interpretation and the effects of low am- rare. These results mean that climate reconstructions based bient CO2 at the LGM which would have altered the relative on terrestrial plant indicators will not be valid for periods dominance of different taxa in a manner that mimics colder with markedly different CO2 levels. and drier climates. Biogeographic and phylogenetic data are cited as independent tests of the CO2 effect model. Correspondence to: C. Loehle The idea that glacial levels of CO2 could affect vegeta- ([email protected]) tion structure and composition was proposed over a decade

Published by Copernicus GmbH on behalf of the European Geosciences Union. 110 C. Loehle: Predicting Pleistocene climate ago (e.g., Beerling, 1996; Beerling and Woodward, 1993; found evidence for temperate forest vegetation right up to Bennet and Willis, 2000; Cole and Monger, 1994; Cowl- the ice margin throughout the period of the last glacial ad- ing, 1999; Cowling and Sykes, 1999; Farquhar, 1997; Harri- vance in , Indiana, and Ohio. Farther north and west, son and Prentice, 2003; Jolly and Haxeltine, 1997; Polley in Minnesota, Montana, and the Dakotas, the southern glacial et al., 1993, 1995; Street-Perrott et al., 1997). Neverthe- margins were frozen to their beds and signs of are less, LGM climate as estimated from pollen data generally present (Clayton et al., 2001; Mickelson et al., 1983). Pew´ e´ still does not factor in CO2 effects (e.g., Elenga et al., 2000; (1983) compiled abundant evidence for permafrost from the Nakagawa et al., 2002; Peyron et al., 1998; Tarasov et al., of and from across Pennsylvania, but 1999) and discussions of simulated LGM climate vs. vege- none from central Illinois east to the edge of Pennsylvania. tation (the anomaly problem) either do not mention the role Pew´ e´ (1983) shows some evidence for patterned ground on of CO2 (e.g., Ganopolski et al., 1998; Pinot et al., 1999) or the high peaks of the Appalachians, but only as far south as only touch on this effect (e.g., Crowley, 2000; Huntley et al., the southern Virginia-West Virginia border. Periglacial forms 2003). south of this either are undated or are not reliably identi- To date, the discussion has featured dueling models. Re- fied (Pew´ e,´ 1983). Denny (1951) documented frost action gression models based on pollen (which seem straightfor- in Pennsylvania that diminishes with distance from the ice ward) are pitted against simulations of plant communities margin. There is thus evidence for a band of permafrost and under low CO2. It is tempting to be wary of the simulations periglacial climate in Pennsylvania and Wisconsin but none because they cannot be directly verified. In what follows, in between. This reconstruction makes sense because the several types of independent evidence are presented that im- area of western Pennsylvania is higher in elevation than are ply that LGM climates inferred from pollen data are colder southern Ohio, Indiana, and Illinois. Thus, permafrost (and than the likely actual climates. These various types of ev- tundra) was not a universal of the ice front region in idence are shown to reconcile with effects of low ambient eastern North America as implied by most reconstructions. CO2, thereby supporting the models of CO2 effects. The re- Wider periglacial zones did of course occur in . sults of the analysis also on questions of glacial refu- The lack of tundra puts limits on how cold this region gia. The focus of this study is largely eastern North America could have been. for the sake of concreteness, but no-analog vegetation found elsewhere at the LGM can be explained by similar mecha- nisms. 3 Taxonomic anomalies

2 Geologic anomalies A key species in the reconstruction of LGM climates in east- ern North America is spruce (Picea). The pollen of various By analogy with European mountains, it has been assumed species of spruce are very difficult to distinguish. Since all (e.g., Deevey, 1949; Delcourt and Delcourt, 1984) that the existing spruce species in eastern North America require a southern Appalachians, particularly the Great Smoky Moun- cold climate, they have generally been considered together tains, should have been covered by permanent ice caps and (see, e.g., Davis and Shaw, 2001). In this context, pollen should have generated . Mountain glaciers produce profiles dominated by spruce and herbaceous species have obvious signs like Ushaped valleys, striations, and . been classified as boreal parkland. None of these signs has ever been found in the southern Ap- It is now apparent, however, that a major portion of the palachians south of Pennsylvania. More recent treatments southern range of spruce may have been occupied by a now- assume tundra but not glaciers on the peaks. extinct species (Jackson and Weng, 1999) which was com- Tundra produce distinctive signs, due to processes mon in the lower and east at least to west- such as solifluction, cryoturbation, ice wedges, and stone ern Georgia. Given that fossil stumps of this species can be sorting. While such disturbances can coexist with trees found growing with and with strictly southern species, (as in ) the lack of such signs rules out permafrost and this was likely a temperate spruce. This spruce pollen is thus therefore contradicts the existence of tundra. No such signs not necessarily indicative of cold climates. This may have have been found more than a few miles south of the edge biased climate reconstructions at the LGM to a considerable of the ice between Wisconsin and Pennsylvania. For exam- degree. ple, Braun (1951) noted that at a site only 15 miles south A similar problem occurs with sedges (Carex). This taxo- of the ice margin in Ohio, evidence of congeliturbation, al- nomic group has been interpreted as diagnostic of boreal or though present, did not indicate severe frost action, an obser- tundra habitats. However, sedges are also common in grass- vation also supported by Wolfe (1951). Black (1976) con- lands and forests and would have been favored under a low cluded that there is no evidence for permafrost in Illinois, CO2 atmosphere (Beerling, 1996; Beerling and Woodward, Indiana, and Ohio south of the ice margin. A number of 1993; Jolly and Haxeltine, 1997; Robinson, 1994; Saxe et sources (Burns, 1958; Gooding, 1963; Gruger,¨ 1972a, b) al., 1998), as will be discussed below.

Clim. Past, 3, 109–118, 2007 www.clim-past.net/3/109/2007/ C. Loehle: Predicting Pleistocene climate 111

In both cases, the species have been considered key in- A major phylogeographic study (Soltis et al., 2006) shows dicators or diagnostic species of boreal forest but lumping that there is a lack of genetic structure concordance across species is known to produce misleading indicators of climate taxa. The genetic structure of a species defines how unique (Finkelstein et al., 2006). The fact that their pollen is found alleles vary over space. Concordance of pattern should occur mixed with that of temperate species such as oaks across the if species shared a common refuge, as did occur in Europe “boreal parkland” (e.g., Webb et al., 1988, 1993, 1997), fur- where concordance is more evident. ther calls into question their indicator status. When models While it is evident that some species did retreat to the Gulf are created for climate, the climatic tolerance of individual Coast (Soltis et al., 2006), there is no evidence for a compre- species is used to create a regression model that predicts cli- hensive southern refuge for the bulk of the species. mate, or a biome is defined from the dominant species, and A related fact is that the high elevation rock outcrop flora that biome is used as an indicator of climate. Inclusion of am- of the southern Appalachians would be expected to be de- biguous genera such as spruce in the model will create a cold rived from boreal/alpine elements that would have moved bias. This analysis also helps explain part of the no-analog in during the LGM. Instead, Wiser (1998) found very lit- vegetation situation. tle overlap between the flora in the high elevation south- ern Appalachians and high elevation flora in the Presidential Range mountains of New Hampshire and the White Moun- 4 Biogeographic anomalies tains, while the latter do show the expected affinities to alpine/tundra floras. The geographic distributions of plants can reveal a great deal about the of a region. This is true of evidence for 4.2 Genetic gradients refugia, genetic gradients, distributions of endemics, and the existence of races and subspecies. After the last glacial advance 18 000 years ago, and the be- ginning of the , the North American began 4.1 Glacial refuge anomalies a rapid retreat (Pielou, 1991; Tallis, 1991). Plants coloniz- ing the previously glaciated lands could potentially spread The traditional model assumes that temperate forest was rapidly over a very large newly deglaciated area. Such forced to retreat to the far south (Gulf Coast region) by rapid spread causes particular effects on population genetic the cold (e.g., Bennett, 1985; Cain et al., 1998; Delcourt structure. If a species was confined to a very small refuge and Delcourt, 1984, 1993; Watts and Stuvier, 1980). He- with a small population, a genetic bottleneck combined with witt (2004) posits an Appalachian refuge, but without cit- homogenization will produce a population with very little ing any data. When we look for fossil evidence for the mi- genetic differentiation (Hewitt, 2004). As this population gration of midwestern temperate forest species to the Gulf spreads across the deglaciated zone, the genetic uniformity Coast and Florida (the refugia for deciduous forest in the tra- will be maintained for a long time. For trees, only a few ditional model), we encounter a striking lack of evidence. hundred generations have passed since the LGM, too little Lakes in the Gulf Coast area, Texas, and Florida either do for either random mutation or local adaptation to have had not date back far enough (i.e., not as far as the LGM) or much effect. An example of this phenomenon is red pine (Pi- show a continuous presence of -pine associations (includ- nus resinosa), which occurs largely in previously glaciated ing such shrubs as wax myrtle at Sheelar Lake) similar to terrain and which has very low heterozygosity (Burns and those present today (Bryant, 1977; Bryant and Holloway, Honkala, 1990). Almost no species can be found matching 1985; Delcourt and Delcourt, 1993; Givens and Givens, this pattern south of the ice sheets (Soltis et al., 2006), sug- 1987; Jackson and Givens, 1994; Watts, 1973; Watts and gesting that species during the Pleistocene were not confined Stuvier, 1980). The most recent pollen maps do not show to small refugia. any distinct refugia for northern temperate species, most of If, on the other hand, a species invades the deglaciated which are either undetectable or diffusely rare across the re- zone from a wide front with a large initial population and gion (Williams, et. al. 2004). It has been shown (Froyd, high heterozygosity, the rapid migration will cause a gradual 2005; McLachlan and Clark, 2004) that rare species can be loss of rare alleles as the distance from the refugial popula- hard to detect in pollen samples. Thus, data do not support tion increases (Hewitt, 2004). This pattern is seen in lodge- the existence of a Florida or Gulf Coast refuge. The area in pole pine (Pinus contorta), which has high heterozygosity question consists of the geographically unbroken piedmonts in the unglaciated refugia and progressive decrease in het- and coastal plains of Georgia, South Carolina, Florida and erozygosity with distance north (Burns and Honkala, 1990). Alabama. Except for fish, there are no real barriers keeping The same genetic structure is seen in black spruce (Picea species from moving from east to west across these areas, mariana) in the formerly glaciated zone, in which north- and they indeed today share a common flora. There is no ernmost populations have low mitochondrial DNA diversity mechanism to create distinct refuges, as there was in Europe. (Gamache et al., 2003). www.clim-past.net/3/109/2007/ Clim. Past, 3, 109–118, 2007 112 C. Loehle: Predicting Pleistocene climate

In contrast to these expectations for rapidly migrating Maps of the distribution of endemic species show that re- populations, species in the unglaciated (but supposedly cently glaciated regions lack endemics (Estill and Cruzan, Pleistocene-era tundra or boreal zone) East show deep DNA 2001; Qian, 1999; Schofield, 2004). The high frequency of divergence (which means long isolation) between separate endemics in central led Hulten´ (1937) to propose that sub-populations, weak or absent genetic gradients, and high a large portion of remained unglaciated and served genetic richness (Hewitt, 2004; Soltis et al., 2006). If these as a refuge, a view since substantiated by geological stud- species had been excluded by cold from the area south of the ies. As Braun (1950, 1951) noted, the southern Appalachi- Great Lakes (southern Appalachians) and migrated back into ans, particularly the region of the Cumberland Plateau and this region, genetic diversity gradients should exist but they the Blue Ridge escarpment, is a center of both species rich- do not. ness and endemism for the eastern United States. Both occur because this region has an ancient floristic history (since at 4.3 Distribution of endemics least middle time) as the only area of large moun- tains in the East. The mountains have a wide variety of soils, climates (often moist and equable), and topographies If tundra and boreal forest/parkland formed zones south of that both provide habitat for many species and encourage en- the ice margin in the East, then temperate vegetation would demism by imposing barriers that isolate populations. Many have had to retreat to the far south to refuges, as is commonly endemics can be found in this region across life forms includ- assumed. The data on endemism seriously constrain theories ing trees (Abies fraseri), ferns, shrubs, herbaceous plants, proposing large-scale migrations of vegetation at the LGM. fish (Gilbert, 1987), salamanders, crayfish, and centipedes. Many endemics occur just south of the ice margin; almost The extremely limited distributions of many endemics in the none are found within the glaciated area (e.g., Baskin and southern Appalachians pose a real difficulty for any theory Baskin, 1986, 1989; Estill and Cruzan, 2001; Qian, 1999; of local extinctions, distant refuges, and subsequent reimmi- Schofield, 2004), and those that do generally are either hy- gration, as does evidence concerning the time of their origin. brids or are recently derived. The only possible explana- Given the large number of endemics, such a major forced mi- tion for this observation is that the unglaciated East had cli- gration should have caused many extinctions, but very few mates that these endemic species could tolerate throughout Pleistocene extinctions of land plants in eastern North Amer- the . ica have been documented (Potts and Behrensmeyer, 1992; Local endemics are species or discrete races with a re- Roy et al., 1996). As a group, these endemics exhibit many stricted geographic distribution (Brown and Gibson, 1983; signs of ancient origin, including high morphologic diver- Cain, 1944). Endemics may arise in a number of ways. gence from related species, geographic isolation from con- In some cases, a species that is originally widespread may generics, complete reproductive isolation, and edaphic spe- become progressively restricted to a unique habitat as cli- cialization. A number of endemic plants, for example, have mate changes, physiography changes, or other species out- no close relative in the East, and their nearest congener is in compete it. It thereby becomes a species. For example, China or Europe (e.g., Qian, 1999). The very few endemics a swamp-dwelling species might progressively lose habitat in glaciated regions do not exhibit these signs. during periods of uplift when swamps are drained. A telling example of the significance of endemics is a com- A second type of origin of endemics involves evolution munity centered on sinkhole ponds in the Blue Ridge Moun- in a unique habitat, leading to the generation of a race, a tain region of Virginia (Church et al., 2003). In this small subspecies, and ultimately a new species. For example, in 1350 ha area are found disjunct and endemic populations of the tropics the species on isolated volcanic peaks are often 70 plants and animals. The adaptation of these species to this derived from tropical lowland elements and are unrelated unique habitat would prohibit these species from migrating to those on other mountains. Striking examples occur in under a changing climate. populations of animals, with the cave population diverg- We are led to the conclusion that the endemics in the ing drastically in morphology from the normal population. unglaciated region have origins that predate the LGM and How many specialized species could have evolved since the therefore that they survived the glacial climate in the loca- LGM is unknown, but data on speciation rates from the fos- tions where they are found, whereas endemics in glaciated sil record (Levin and Wilson, 1976) suggest that very few regions have a recent origin. species could have originated in such a short time. This is especially true for trees, which have only experienced a few 4.4 Distribution of races and subspecies hundred generations since the LGM. Telltale signs of recent origin for endemics are the proximity of the parent species, The geographic distribution of races and subspecies provides incomplete genetic isolation, and limited morphologic diver- invaluable information for interpreting past climatic changes. gence. These signs of recent origin do not apply to most en- Even though most race and subspecies distinctions cannot be demics in the unglaciated regions but do apply to those found made using pollen data, such distinctions cannot be ignored. in the recently glaciated regions. If multiple distinct races of a species can be identified, then

Clim. Past, 3, 109–118, 2007 www.clim-past.net/3/109/2007/ C. Loehle: Predicting Pleistocene climate 113

(1) each must have had a separate glacial refuge to prevent (Parker et al., 1997). If Virginia pine had moved down introgression; or (2) they all evolved since the LGM; or (3) into Florida at the LGM, it would have merged with this the bulk of the range of the species was not disturbed during closely related species. These examples are just a sample the glacial period. If they had all been jammed together in a of the many cases where closely related species are today single refuge, they would have merged into a single species. prevented from crossing by geographic separation but would Because, as noted, finding even a single glacial refuge in the have merged if crowded together in a single coastal plain far south has proven problematic, finding multiple refugia refuge. The races of these several species do not coincide within which each of the distinct races of these many species geographically, further multiplying the number of required could be preserved would seem doubly problematic. This is refuges. Phylogenetic analysis of these species leads to the particularly true due to the geography of eastern North Amer- same conclusion (Soltis et al., 2006). ica. The Gulf Coastal Plain lacks significant east-west bar- riers, so most plant species today occur across most of the plain if they occur there at all. There is no mechanism by 5 Discussion which different races or related species of plant could be ge- ographically isolated during the Pleistocene in this region. The anomalies documented here are not trivial. The biogeo- Fagus grandifolia has three distinct races, the gray, red, graphic patterns expected for rapidly migrating species are and white (Bennett, 1985; Camp, 1950; Cooper and observed in the glaciated zone, but rarely in the unglaciated Mercer, 1977), which differ on multiple traits. These three zone. Boreal zone species exhibit low genetic variation or races readily hybridize where their ranges overlap. in diversity gradients, but species in the unglaciated zone show formerly glaciated territory appear to originate from popula- high genetic richness, no gradients, and deep (ancient) DNA tions west of the and just south of divergence between populations. Endemics are very rare the (McLachlan et al., 2005). Florida red maple in the glaciated zone, and endemics in this zone appear to (Acer rubrum) is genetically distinct from other red maple be mostly recent (due to hybridization, for example). Sis- populations (McLachlan et al., 2005). Genetic data for red ter species, subspecies, and races are common across the maple further support refugia close to the ice (Soltis et al., unglaciated East but not in the glaciated zone. To keep all 2006). Evidence is also accumulating for southern Ap- of these groups separate during the Pleistocene so that they palachian refugia for various animal species (e.g., Austin did not hybridize/homogenize out of existence would require et al., 2002, 2004; Brant and Ort´ı, 2003; Church et al., 2003). multiple LGM refuges, but no such refuges have been found The genetics data for Liriodendron point to a number of dis- in the far South. If the pollen data say one thing and the bio- tinct races. Virginia pine has two distinct races, as defined geographic data flatly contradict this inference, what can we by genetics data (Parker et al., 1997); one to the northwest conclude? and one to the southeast of the main Appalachian axis. Al- There is a possible resolution to this paradox based on cli- though Parker et al. (1997) attribute the origin of the north- mate and CO2 factors. First, glacial climates are not simply a western population to a postglacial migration from the south- general cooling of the climate. Glacial climates were east, based on its lower genetic diversity (often found in col- cooler, but more so in summer (Pielou, 1991). Thus, the ex- onizing populations), the substantial number of unique alle- istence of “boreal” species farther south than today does not les found in various northwestern populations suggest that mean that climates were bitterly cold. Second, a factor that it is not merely a genetic subset of the southeastern popula- also varied during the glacial period is CO2, which was so tion. Rather, this gives a suggestion of prolonged isolation, low that it caused CO2 . CO2 starvation does not possibly throughout the Pleistocene, in which case it had to affect all species equally. remain to the northwest of the mountains during the LGM to At the LGM, about 18 000 years ago, CO2 levels were remain separate from the southeastern race. Fraser fir, found very low, less than 200 ppm. This low level of CO2 con- in the southern Appalachians, will hybridize with balsam fir, stitutes a severe deficiency for growth, and may have shifted but the only hybrids at present are near West Virginia. In competitive dominance between different plant types, as all of these cases the distinct races would have required the as affecting overall vegetation biomass (Levis and Foley, existence of multiple refuges in order to persist. 1999). Altered CO2 levels could also affect pollen produc- We may similarly look at closely related species for evi- tion, which would bias biome reconstruction. C4 grasses dence of LGM distributions. Sugar maple and Florida maple have a strong advantage over other plants under low CO2 are not well separated morphologically. Where their ranges (Cole and Monger, 1994; Farquhar, 1997; Polley et al., 1993, overlap, they hybridize readily. If sugar maple had migrated 1995), though not under low temperatures. Robinson (1994) to the Gulf Coast, it would have occupied the same range shows that the extent of stomatal regulation plants exhibit in as Florida maple, in which case the two species would have response to CO2 level varies by taxa in a manner that sug- merged. A similar problem exists with Virginia pine. This gests that this trait is a modern adaptation. Ancient taxa such pine is only kept from crossing with Pinus clausa (a Gulf as conifers exhibit little stomatal responsiveness compared to Coast species) by their considerable geographic separation angiosperms. The benefit of stomatal responsiveness trades www.clim-past.net/3/109/2007/ Clim. Past, 3, 109–118, 2007 114 C. Loehle: Predicting Pleistocene climate off against greater water loss at low CO2 levels resulting from with low levels of tropical montane forest pollen persisting stomates being open longer (Drake et al., 1997; Saxe et al., in LGM profiles dominated by ericaceous shrubs. While 1998). Where water is not limiting, a cool low CO2 climate drought-tolerant boreal elements such as white spruce (or should favor C3 grasses, forbs and angiosperm trees over the extinct temperate Picea crutchfieldii) could have moved conifers (Beerling, 1996; Beerling and Woodward, 1993; south under a cooler, low CO2 climate, most eastern boreal Jolly and Haxeltine, 1997; Robinson, 1994; Saxe et al., trees are not drought tolerant and would not have been able to 1998). This suggests that mesic broadleaf trees should have move far south. We observe exactly this pattern of change in remained competitive in mesic microsites during the Pleis- the pollen record, with most boreal trees being absent from tocene. On most upland sites, however, a low CO2 climate the supposed boreal parkland south of the ice. Several au- would favor xeric species such as conifers, which have con- thors have proposed that the southern Appalachians were a sistently low stomatal conductance, and herbaceous species, refuge (e.g., Braun, 1950; 1951; Church et al., 2003; Harvill, including grasses and sedges. Oaks, as trees with intermedi- 1973; Hewitt, 2004). This view now has increased plausibil- ate responsiveness, would have persisted as well on upland ity. This analysis provides independent biogeographic sup- sites. We in fact observe extensive open conifer forest re- port for model-based projections of the effects of CO2 on placement of broadleaf forest at the LGM in eastern North LGM vegetation. Other factors such as altered fire regimes America, with a persistent oak component. Simulation stud- and could also impact the details of reconstruc- ies also show that the reduced water use efficiency expected tions. at the LGM would produce a xeric, open forest (with low leaf The above discussion has implications for the locations of area index) south of the ice dominated by conifers (Cowl- refugia in Europe. The traditional reconstruction of LGM ing, 1999; Harrison and Prentice, 2003; Jolly and Haxeltine, Western Europe has been a treeless steppe-tundra as far south 1997; Levis and Foley, 1999). This explains the “no-analog” as southern (Stewart and Lister, 2001), with presumed open forest (or parkland) often remarked upon. This discus- refugia in the Mediterranean areas such as the Iberian and sion shows that species that currently occur together would Italian peninsulas (Stewart and Lister, 2001). However, fos- diverge in their responses under the LGM climate, and that sils of thermophilous trees and have been dated to climate predictions based on their occurrence would not be the LGM in Belgium, England, Hungary, Slovakia, and Ger- correct. Exact simulations of vegetation would need to be many, among other places (Stewart and Lister, 2001; Willis based on more detailed studies of the physiology of each and van Andel, 2004). In addition, populations of numer- species, few of which have been done. ous trees and animals such as Scots pine in (Stew- Simulations of the effect of low CO2 levels show that it art and Lister, 2001) and hedgehog in Germany (Willis and could cause a major lowering of alpine treelines (Bennett and Whittaker, 2000) are either genetically distinct from Mediter- Willis, 2000; Cowling and Sykes, 1999; Street-Perrott et al., ranean populations (Scots pine) or have no southern relatives 1997). Jolly and Haxeltine (1997) simulated the montane (hedgehog). For these species, it appears that they persisted ecotone for African mountains and showed that the entire throughout the LGM in refugia scattered across Europe in LGM lowering of treeline at their study site is consistent with what is assumed to be a climate too cold for them to per- the effect of low CO2 without assuming any drop in temper- sist. Stewart and Lister (2001) interpret these refugia, most ature. Lower treelines at the LGM have typically been taken of those known being from cave sites in steep valleys, as evidence for colder temperatures, but could really be the as thermal refuges from the cold. However, steep val- result of changes in CO2 levels. They would thus not corre- leys are not particularly known for providing thermal refuges spond to snowline depressions (e.g., Greene et al., 2002) or in present-day northern habitats. In fact, in hilly terrain, it is other indicators of temperature. south and west facing midslopes and ridges that are warmest, If lowered CO2 affected vegetation in eastern North Amer- not valleys, which receive less sun and are subject to cold air ica, what changes would be expected? Grasses and other drainage. It is more likely that these valleys provided mesic species would be expected to expand their range refuges from the combined effects of a drier climate and the into forest, perhaps creating parkland. Forest communi- reduced water use efficiency caused by the CO2 effect. These ties would shift to drought tolerant species such as mesic plant communities would provide a home for the - and oaks. Significantly, mesic microsites such as coves, mals found there. Just such steep valley and stream-margin north slopes, and stream valleys would provide refuges for refuges are found today in dry regions across the world. It mesic species across their entire original range, especially is noteworthy that tree species that went extinct in Europe in the highly dissected Appalachian region, because the ef- were less drought tolerant than surviving species (Svenning, fect of CO2 would be similar to an overall drying. High 2004), as predicted by the CO2 effect model. To an even precipitation, midelevation regions such as the Cumberland greater extent than in North America, the reduction of tree Plateau and the Southern Blue Ridge Escarpment would pro- cover due to the CO2 effect may have given a false impres- vide prominent refuges. In contrast, topography provides sion of extreme dryness in Europe. It is not asserted that all much less protection against extreme cold. An LGM African aspects of the situation are analogous. montane site (Jolly and Haxeltine, 1997) shows this pattern, There are a number of implications of these results for

Clim. Past, 3, 109–118, 2007 www.clim-past.net/3/109/2007/ C. Loehle: Predicting Pleistocene climate 115 the practice of climate reconstruction and for testing cli- acted as a refuge for species of the eastern deciduous for- mate models against historical proxy data. Regression or est. This explains the almost complete lack of tree species response surface approaches (e.g., Farrera et al., 1999; Nak- extinctions in this region. It also suggests that the use of agawa et al., 2002; Peyron et al., 1998; Tarasov et al., 1999; plant remains to predict climate for any period of the past in Web et al., 1993, 1997) implicitly assume that species’ cli- which CO2 level was appreciably different from today may mate responses are stable over time, but the CO2 effect model lead to incorrect conclusions unless the effect of CO2 on rel- suggests that this assumption is violated. The use of plant ative growth rates is accounted for. functional types to reconstruct either biomes (e.g., Elenga et al., 2000) or climate (e.g., Peyron et al., 1998; Tarasov Acknowledgements. This work was sponsored by the U.S. De- et al., 1999) has the same issues as response surface mod- partment of Energy, Office of Energy Research, Office of Health els and also assumes that similar species will respond to a and Environmental Research, Program for Research, changing climate as a group. It was shown above that nor- under contract W-31-109-Eng-38 and by the National Council for mally co-occurring species will show divergent responses to Air and Stream Improvement, Inc. Some of the concepts about endemic distributions and difficulties with the pollen record have CO change. Very few studies can be found that take explicit 2 been expressed in correspondence and talks by H. Iltis dating back account of CO2 effects when reconstructing vegetation and to the 1960s, including a talk in 1994 at the Ecological Society of climate (e.g., Guiot et al., 2000; Levis et al., 1999; Jolly and America meeting in Knoxville, TN. Helpful reviews were provided Haxeltine, 1997). Regression and response surface models, by H. Birks, M. Davis, J. Hoffecker, H. Iltis, R. K. Peet, J. Ritchie, including models of plant functional types, also are unable J. Tallis, and T. Webb III. H. Iltis provided many references. to account for other changes in the conditions at the LGM. For instance, seasonal distributions of temperature were not Edited by: C. Hatte´ strictly analogous to latitudinal shifts. That is, the colder temperatures at the LGM did not produce seasonal shifts that correspond to a simple northward movement of place. This throws off calibration of models of plant response to climate. References Annual variability may have been altered, fire regimes may have been different, and in North America the presence of Austin, J. D., Lougheed, S. C., Neidrauer, L., Chek, A. A., and large herbivores that later went extinct really should not be Boag, P. T.: Cryptic lineages in a small frog: The post-glacial his- tory of the spring peeper, Pseudacris crucifer (Anura: Hylidae), ignored. All of these factors could have shifted the commu- Molecular Phylogenetics and Evolution, 25, 316–329, 2002. nity composition in ways that would give the impression of Austin, J. D., Lougheed, S. C., and Boag, P. T.: Discordant tem- some climate effect. poral and geographic patterns in maternal lineages of eastern Testing of general circulation models is likewise affected North American frogs, Rana castebeiana (Ranidae) and Pseu- by the CO2 response of vegetation. While there is some dacris crucifer (Hylidae), Molecular Phylogenetics and Evolu- recognition that CO2 could affect pollen interpretations, the tion, 32, 799–816, 2004. range of weight given to this issue ranges from none (Nak- Baskin, J. M. and Baskin, C. C.: Distribution and geographi- agawa et al., 2002; Pinot et al., 1999) to a consideration cal/evolutionary relationships of cedar glade endemics in south- eastern United States, ASB Bulletin, 33, 138–154, 1986. of only the C3 vs. C4 effect (Crowley, 2000; Farrera et al., 1999). Few climate model comparisons to paleotemperature Baskin, J. M. and Baskin, C. C.: Cedar glade endemics in Ten- proxies have fully factored in the CO effects on leaf area, nessee, and a review of their autecology, J. Tennessee Academy 2 Sci., 64, 63–74, 1989. plant types, and water use efficiency in the temperature re- Beerling, D. J.: Ecophysiological responses of woody plants to past construction. The treatment of vegetative cover and transpi- CO2 concentrations, Tree Physiology, 16, 389–396, 1996. ration rate within climate simulation models is probably also Beerling, D. J. and Woodward, F. I..: Ecophysiological responses deficient in this regard, though published descriptions of this of plants to global environmental change since the Last Glacial model component are not usually adequate to determine ex- Maximum, New Phytologist, 125, 641–648, 1993. actly how vegetation is modeled. Bennett, K. D.: The spread of Fagus grandifolia across eastern In summary, the entire interpretation of LGM vegetation North America during the last 18 000 years, J. Biogeogr., 12, and climate in eastern North America may have been bi- 147–164, 1985. Bennett, K. D. and Willis, K. J.: Effect of global atmospheric carbon ased by the CO2 effect. It was neither as cold nor as dry as has been assumed. Lower treelines were probably caused dioxide on glacial- vegetation change, Global Ecol. by the CO effect. The “no-analog” conifer woodlands are Biogeogr., 9, 355–361, 2000. 2 Black, R. F.: Periglacial features indicative of permafrost: Ice and the direct result of changes in water use efficiency between soil wedges, Res., 6, 3–26, 1976. taxa and the lumping of spruce species and sedge species Bradley, R. S.: : Reconstructing climates of the into generic-level categories. The presence of spruce in east- Quaternary, Second edition, Elsevier Academic Press, Amster- ern North America was not an indicator of a boreal climate. dam, 1999. Massive vegetation dislocations and migrations did not oc- Brant, S. V. and Ort´ı, G.: of the northern short- cur prior to ice melt, and the entire LGM unglaciated region tailed shrew, Blarina brevicauda (Insectivora: Soricidae): Past www.clim-past.net/3/109/2007/ Clim. Past, 3, 109–118, 2007 116 C. Loehle: Predicting Pleistocene climate

fragmentation and postglacial recolonization, Molecular Ecol., Delcourt, H. R. and Delcourt, P. A.: Ice age haven for hardwoods, 12, 1435–1449, 2003. Natural History, 84, 22–28, 1984. Braun, E. L.: Deciduous forests of eastern North America, Hafner Delcourt, P. A. and Delcourt, H. R.: Paleoclimates, paleovegeta- Press, London, 1950. tion, and paleofloras during the late Quaternary, 71–94 in Flora of Braun, E. L.: Plant distribution in relation to the glacial boundary, North America Editorial Committee (ed.), Flora of North Amer- Ohio J. Sci., 51, 139–146, 1951. ica, Oxford University Press, New York, 1993. Brown, J. H. and Gibson, A. C.: Biogeography, C. V. Mosby Co., Denny, C. S.: Pleistocene frost action near the border of the Wis- St. Louis, MO, 1983. consin drift in Pennsylvania, Ohio J. Sci., 51, 116–125, 1951. Bryant, V.M., Jr.: A 16,000 pollen record of vegetation change Drake, B. G., Gonzalez-Meler,` M. A., and Long, S. P.: More effi- in central Texas, , 1, 143-156, 1977. cient plants: A consequence of rising atmospheric CO2?, Ann. Bryant, V. M. and Holloway, R. G.: A late Quaternary paleoenvi- Rev. Plant Physiol. Plant Molecular Biol., 48, 609–639, 1997. ronmental record of Texas: An overview of the pollen evidence, Elenga, H., Peyron, O., Bonnefille, R., Jolly, D., Cheddadi, R., 39-70 in V.M. Bryant and R.G. Holloway (eds.), Pollen records Guiot, J., Andrieu, V., Bottema, S., Buchet, G., de Beaulieu, J.- of late Quaternary North American , American Asso- L., Hamilton, A.C., Maley, J., Marchant, R., Perez-Obiol, R., ciation of Stratigraphic Palynologists Foundation, Dallas, 1985. Reille, M., Riollet, G., Scott, L., Straka, H., Taylor, D., Van Burns, G. W.: Wisconsin age forests in western Ohio: II, Vegetation Campo, E., Vincens, A., Laarif, F., and Jonson, H.: Pollen-based and burial conditions, Ohio J. Sci., 58, 220–230, 1958. biome reconstruction for southern Europe and Africa 18 000 yr Burns, R. M. and Honkala, B. H.: Silvics of North America, Vol. 1, bp, J. Biogeogr., 27, 621–634, 2000. Conifers, USDA Handbook 654, 1990. Estill, J. C. and Cruzan, M. B.: Phytogeography of rare plant species Cain, S. A.: Foundations of plant geography, Harper & Row, New endemic to the southeastern United States, Castanea, 66, 3–23, York, 1944. 2001. Cain, M. L., Damman, H., and Muir, A.: Seed dispersal and the Farquhar, G. D.: Carbon dioxide and vegetation, Science, 278, Holocene migration of woodland herbs, Ecol. Monogr., 68, 325– 1411, 1997. 347, 1998. Farrera, I., Harrison, S. P., Prentice, I. C., Ramstein, G., Guiot, J., Camp, W. H.: A biogeographic and paragenetic analysis of the Bartlein, P. J., Bonnefille, R., Bush, M., Cramer, W., von Grafen- American beech (Fagus), 166–169 in Yearbook of the American stein, U., Holmgren K., Hooghiemstra, H., Hope, G., Jolly, D., Philosophical Society, 1950. Lauritzen, S.-E., Ono, Y., Pinot, S., Stute, M., and Yu, G.: Trop- Church, S. A., Kraus, J. M., Mitchell, J. C., Church, D. R., and Tay- ical climates at the Last Glacial Maximum: A new synthesis of lor, D. R.: Evidence for multiple Pleistocene refugia in the post- terrestrial palaeoclimate data. I. Vegetation, lake-levels and geo- glacial expansion of the eastern salamander, Ambystoma chemistry, Clim. Dyn., 15, 823–856, 1999. tigrinum tigrinum, Evolution, 57, 372–383, 2003. Finkelstein, S. A., Gajewski, K., and Viau, A. E.: Improved res- Clayton, L., Attig, J. W., and Mickelson, D. M.: Effects of late olution of pollen taxonomy allows better biogeographical inter- Pleistocene permafrost on the landscape of Wisconsin, USA, pretation of post-glacial forest development: Analyses from the Boreas, 30, 173–188, 2001. North American Pollen Database, J. Ecol., 94, 415–430, 2006. Cole, D. R. and Monger, H. C.: Influence of atmospheric CO2 on Froyd, C. A.: Fossil stomata reveal early pine presence in Scotland: the decline of C4 plants during the last , , 368, Implications for postglacial colonization analyses, Ecology, 86, 533–536, 1994. 579–586, 2005. Cooper, A. W. and Mercer, E. P.: Morphological variation in Fagus Gamache, I., Jaramillo-Correa, J. P., Payette, S., and Bousquet, J.: grandifolia Ehrh. in North Carolina, Journal of Elisha Mitchell Diverging patterns of mitochondrial and nuclear DNA diversity Scientific Society, 93, 136–149, 1977. in black spruce: Imprint of a founder effect associ- Cowling, S. A .: Simulated effects of low atmospheric CO2 on ated with postglacial colonization, Molecular Ecol., 12, 891– structure and composition of North American vegetation at the 901, 2003. Last Glacial Maximum, Global Ecol. Biogeogr., 8, 81–93, 1999. Ganopolski, A., Rahmstorf, S., Petoukhov, V., and Claussen, M.: Cowling, S. A. and Sykes, M. T.: Physiological significance of Simulation of modern and glacial climates with a coupled global low atmospheric CO2 for plant-climate interactions, Quaternary model of intermediate complexity, Nature, 391, 351–356, 1998. Res., 52, 237–242, 1999. Gilbert, C. R.: Zoogeography of the freshwater fish fauna of south- Crowley, T. J.: CLIMAP SSTs re-visited, Clim. Dyn., 16, 241–255, ern Georgia and peninsular Florida, Brimleyana, 13, 25–54, 2000. 1987. Davis, M. B.: Quaternary history of deciduous forests of eastern Givens, C. R., and Givens, F. M.: Age and significance of North America and Europe, Annals of the Missouri Botanical fossil white spruce (Picea glauca), Tunica Hills, - Garden, 70, 550–563, 1983. Mississippi, Quaternary Res., 27, 283–296, 1987. Davis, M. B.: Holocene vegetational history of the eastern United Gooding, A. M.: and Wisconsin glaciations in the White- States, 166–181, in: Late-Quaternary environments of the United water basin, southeastern Indiana, and adjacent areas, J. Geol., States, edited by: Wright Jr., H. E., Vol. 2, The Holocene, Long- 71, 665–682, 1963. man, London, 1984. Greene, A. M., Seager, R. and Broecker, W. S.: Tropical snow- Davis, M. B. and Shaw, R. G.: Range shifts and adaptive responses line depression at the Last Glacial Maximum: Comparison with to quaternary , Science, 292, 673–679, 2001. proxy records using a single-cell tropical climate model, J. Geo- Deevey Jr., E. S.: Biogeography of the Pleistocene, Geological Soc. phys. Res., 107(D8), doi:10.1029/2001JD000670, 2002. Am. Bull., 60, 1315–1416, 1949. Gruger,¨ E.: Late Quaternary vegetational development in south-

Clim. Past, 3, 109–118, 2007 www.clim-past.net/3/109/2007/ C. Loehle: Predicting Pleistocene climate 117

central Illinois, Quaternary Res., 2, 217–231, 1972a. 139–147, 2004. Gruger,¨ E.: Pollen and seed studies of Wisconsinan vegetation in McLachlan, J. S., Clark, J. S., and Manos, P. S.: Molecular in- Illinois, USA, Geological Soc. Am. Bull., 83, 2715–2734, 1972b. dicators of tree migration capacity under rapid climate change, Guiot, J., Torre, F., Jolly, D., Peyron, O., Boreux, J. J., and Ched- Ecology, 86, 2088–2098, 2005. dadi, R.: Inverse vegetation modeling by Monte Carlo sampling Mickelson, D. M., Clayton, L., Fullerton, D. S., and Borns Jr., H. to reconstruct palaeoclimates under changed precipitation sea- W.: The late Wisconsin glacial record of the sonality and CO2 conditions: application to glacial climate in in the United States, 3-37 in: Late-Quaternary environments of Mediterranean region. Ecol. Modell., 127, 119–140, 2000. the United States, edited by: Wright Jr., H. E., Vol. 1, The late Harrison, S. P. and Prentice, C. I.: Climate and CO2 controls on Pleistocene, S.C. Porter (Ed.), University of Minnesota Press, global vegetation distribution at the last glacial maximum: Anal- Minneapolis, 1983. ysis based on palaeovegetation data, biome modeling and palaeo- Nakagawa, T., Tarasov, P. E., Nishida, K., Gotanda, K., and Yasuda, climate simulations, Global Change Biol., 9, 983–1004, 2003. Y.: Quantitative pollen-based climate reconstruction in central Harvill Jr., A.M.: Phytogeography of the Virginias and the equilib- Japan: application to surface and Late Quaternary spectra, Qua- rium concept of landscape, Castanea, 38, 266–268, 1973. ternary Sci. Rev., 21, 2099–2113, 2002. Hewitt, G. M.: Genetic consequences of climatic oscillations in the Overpeck, J. T., Webb, R. S., and Webb III, T.: Mapping eastern Quaternary, Philosophical Transactions of the Royal Society of North American vegetation change of the past 18 ka: No-analogs London B, 359, 183–195, 2004. and the , Geology, 20, 1071–1074, 1992. Hulten,´ E.: Outline of the history of Arctic and boreal biota during Parker, K. C., Hamrick, J. L., Parkers, A. J., and Stacy, E. A.: Al- the Quaternary Period, Bokforlags Aktiebolager Thule, Stock- lozyme diversity in Pinus virginiana (Pinaceae): Intraspecific holm, 1937. and interspecific comparisons, Am. J. Botany, 84, 1372–1382, Huntley, B., Alfano, M. J., Allen, J. R. M., Pollard, D., Tzedakis, 1997. P. C., de Beaulieu, J.-L., Gruger,¨ E., and Watts, B.: European Pew´ e,´ T. L.: The periglacial environment in North America during vegetation during Marine -3, Quaternary Wisconsin time, 157–189 in: Late-Quaternary environments of Res., 59, 195–212, 2003. the United States, edited by: Wright Jr., H. E., Vol. 1, The late Jackson, S. T. and Givens, C. R.: Late Wisconsinan vegetation and Pleistocene, S.C. Porter (Ed.), University of Minnesota Press, environment of the Tunica Hills regions, Louisiana and Missis- Minneapolis, 1983. sippi, Quaternary Res., 41, 316–325, 1994. Peyron, O., Guiot, J., Cheddadi, R., Tarasov, P., Reille, M., de Jackson, S. T. and Weng, C.: Late of a tree Beaulieu, Bottema, S., and Andrieu, V.: Climatic reconstruction species in eastern North America, PNAS, 96, 13 847–13 852, in Europe for 18 000 yr B.P. from pollen data, Quaternary Res., 1999. 49, 183–196, 1998. Jacobson Jr., G. L., Webb III, T., and Grimm, E. C.: Patterns Pielou, E. C.: After the Ice Age, University of Chicago Press, and rates of vegetation change during the deglaciation of east- Chicago, 1991. ern North America, 277–288, in: The , Pinot, S., Ramstein, G., Harrison, S. P., Prentice, I. C., Guiot, J., Ruddiman, W. F. and Wright Jr., H. E., Vol. K-3, North America Stute, M., and Joussaume, S.: Tropical paleoclimates at the Last and adjacent oceans during the last deglaciation, Geological Soc. Glacial Maximum: Comparison of Paleoclimate Modeling In- Am., 1987. tercomparison Project (PMIP) simulations and paleodata, Clim. Jolly, D. and Haxeltine, A.: Effect of low glacial atmosphere CO2 Dyn., 15, 857–874, 1999. on tropical African montane vegetation, Science, 276, 786–788, Polley, H. W., Johnson, H. B., Marino, B. D., and Mayeux, H. 1997. S.: Increase in C3 plant water-use efficiency and biomass over Kageyama, M., Peyron, O., Pinot, S., Tarasov, P., Guiot, J., Jous- glacial to present CO2 concentrations, Nature, 361, 61–64, 1993. saume, S., and Ramstein, G.: The Last Glacial Maximum climate Polley, H. W., Johnson, H. B., and Mayeux, H. S.: Nitrogen and wa- over Europe and western : a PMIP comparison between ter requirements of C3 plants grown at glacial to present carbon models and data, Clim. Dyn., 17, 23–43, 2001. dioxide concentrations, Functional Ecol., 9, 86–96, 1995. Levin, D. A. and Wilson, A. C.: Rates of evolution in seed plants: Potts, R. and Behrensmeyer, A. K.: Late terrestrial Net increase in diversity of chromosome numbers and species , 419–436, in: Terrestrial ecosystems through time, numbers through time, Proceedings of the National Academy of edited by: Behrensmeyer, A. K., Damuth, J. D., DiMichele, W. Sciences, 73, 2086–2090, 1976. A., Potts, R., Sues, H.-D., and Wing, S. L., University of Chicago Levis, S. and Foley, J. A.: CO2, climate, and vegetation feedbacks Press, Chicago, 1992. at the Last Glacial Maximum, J. Geophys. Res., 104, 31 191– Prentice, I. C., Bartlein, P. J., and Webb III, T.: Vegetation and cli- 31 198, 1999. mate change in eastern North America since the last glacial max- Maher Jr., L.J., Miller, N. G., Baker, R. G., Curry, B. B., and Mick- imum, Ecology, 72, 2038–2056, 1991. elson, D. M.: Paleobiology of the sand beneath the Valders di- Qian, H.: Spatial pattern of vascular plant diversity in North Amer- amicton at Valders, Wisconsin, Quaternary Res., 49, 208–221, ica north of Mexico and its floristic relationship with , 1998. Ann. Botany, 83, 271–283, 1999. Maxwell, J. A. and Davis, M. B.: Pollen evidence of Pleistocene Ritchie, J. C.: of Canada, Cambridge Univer- and Holocene vegetation on the Allegheny Plateau, Maryland, sity Press, Cambridge, 1987. Quaternary Res., 2, 506–530, 1972. Robinson, J. M.: Speculations on carbon dioxide starvation, late McLachlan, J. S. and Clark, J. S.: Reconstructing historical ranges Tertiary evolution of stomatal regulation and floristic moderniza- with fossil data at continental scales, Forest Ecol. Manage., 197, tion, Plant, Cell Environment, 17, 345–354, 1994.

www.clim-past.net/3/109/2007/ Clim. Past, 3, 109–118, 2007 118 C. Loehle: Predicting Pleistocene climate

Roy, K., Valentine, J. W., Jablonski, D., and Kidwell, S. M.: Scales Watts, W. A.: The late Quaternary vegetation history of the south- of climatic variability and time averaging in Pleistocene biotas: eastern United States, Ann. Rev. Ecol. Syst., 11, 387–409, 1980a. Implications for ecology and evolution, Trends in Ecology and Watts, W. A.: Late Quaternary vegetation history at White Pond on Evolution, 11, 458–463, 1996. the inner coastal plain of South Carolina, Quaternary Res., 13, Royall, P. D., Delcourt, P. A., and Delcourt, H. R.: Late Quaternary 187–199, 1980b. paleoecology and paleoenvironments of the central Mississippi Watts, W. A. and Stuvier, M.: Late Wisconsin climate of northern alluvial valley, Geol. Soc. Am. Bull., 103, 157–170, 1991. Florida and the origin of species-rich deciduous forest, Science, Saxe, H., Ellsworth, D. S., and Heath, J.: Tree and forest func- 210, 325–327, 1980. tioning in an enriched CO2 atmosphere, New Phytologist, 139, Webb III, T., Anderson, K. H., Bartlein, P. J., and Webb, R. S.: Late 395–436, 1998. Quaternary climate change in eastern North America: A com- Schofield, W. B.: Endemic genera of bryophytes of North America parison of pollen-derived estimates with climate model results, (north of Mexico), Preslia Praha, 76, 255–277, 2004. Quat. Sci. Rev., 17, 587–606, 1997. Schoonmaker, P. K. and Foster, D. R.: Some implications of paleoe- Webb III, T., Bartlein, P. J., Harrison, S. P., and Anderson, K. H.: cology for contemporary ecology, Botanical Rev., 57, 204–245, Vegetation, lake levels, and climate in eastern North America for 1991. the past 18 000 years, 415–467, in: Global climates Since the Soltis, D. E., Morris, A. B., McLachlan, J. S., Manos, P. S., and Last Glacial Maximum, edited by: Wright Jr., H. E., Kutzbach, Soltis, P. S.: Comparative phylogeography of unglaciated eastern J. E., Webb III, T., Ruddiman, W. F., Street-Perrott, F. A., and North America, Molecular Ecol., 15, 4261–4293, 2006. Bartlein, P. J., University of Minnesota Press, Minneapolis, 1993. Stewart, J. R. and Lister, A. M.: Cryptic northern refugia and the Webb III, T., Bartlein, P. J., and Kutzbach, J. E.: Climatic change origins of the modern biota, Trends Ecol. Evolution, 16, 608– in eastern North America during the past 18 000 years: Compar- 613, 2001. isons of pollen data with model results, 447–462, in: The geol- Street-Perrott, F. A., Huang, Y., Perrott, R. A., Eglinton, G., Barker, ogy of North America, edited by: Ruddiman, W. F. and Wright P., Khelifa, L. B., Harkness, D. D., and Olago, D. O.: Impact of Jr., H. E., , Vol. K-3, North American and adjacent oceans during lower atmospheric carbon dioxide on tropical mountain ecosys- the last deglaciation, Geological Society of America, 1988. tems, Science, 278, 1422–1426, 1997. Whitehead, D. R.: Late Wisconsin vegetational changes in Svenning, J.-C.: Deterministic Plio-Pleistocene extinctions in the unglaciated eastern North America, Quaternary Res., 3, 621– European cool-temperate tree flora, Ecol. Lett., 6, 646–653, 631, 1973. 2003. Wilkins, G. R., Delcourt, P. A., Delcourt, H. R., Harrison, F. W., Tallis, J. H.: Plant community history, Chapman and Hall, New and Turner, M. R.: Paleoecology of central Kentucky since the York, 1991. Last Glacial Maximum, Quaternary Res., 36, 224–239, 1991. Tarasov, P. E., Peyron, O., Guiot, J., Brewer, S., Volkova, V. S., Williams, J. W., Shuman, B. N., Webb III, T., Bartlein, P. J., and Bezusko, L. G., Dorofeyuk, N. I., Kvavadze, E. V., Osipova, I. Leduc, P. L.: Late-Quaternary vegetation dynamics in North M., and Panova, N. K.: Last Glacial Maximum climate of the for- America: Scaling from taxa to biomes, Ecol. Monogr., 74, 309– mer Soviet Union and Mongolia reconstructed from pollen and 334, 2004. plant macrofossil data, Clim. Dyn., 15, 227–240, 1999. Willis, K. J. and van Andel, T. H.: Trees or no trees? The environ- Watts, W. A.: The full-glacial vegetation of northwestern Georgia, ments of central and eastern Europe during the Last Glaciation, Ecology, 51, 17–33, 1970. Quaternary Sci. Rev., 23, 2369–2387, 2004. Watts, W. A.: Postglacial and interglacial vegetation history of Willis, K. J. and Whittaker, R. J.: The refugial debate, Science, 287, southern Georgia and central Florida, Ecology, 52, 676–689, 1406–1407, 2000. 1971. Wiser, S. K.: Comparison of southern Appalachian high-elevation Watts, W. A.: The vegetation record of a mid-Wisconsin interstadial outcrop plant communities with their northern Appalachian in northwest Georgia, Quaternary Res., 3, 257–268, 1973. counterparts, J. Biogeogr., 25, 501–513, 1998. Watts, W. A.: Late Quaternary vegetation of central Appalachia and Wolfe, J. N.: The possible role of microclimate, Ohio J. Sci., 51, the New Jersey coastal plain, Ecol. Monogr., 49, 427–469, 1979. 134–138, 1951.

Clim. Past, 3, 109–118, 2007 www.clim-past.net/3/109/2007/